Heretofore, a polyester resin such as a polyethylene terephthalate resin has been widely used for packaging containers for various beverages, etc., since it is excellent in mechanical strength, chemical stability, gas barrier property, aroma-retention property, hygienics, etc., and is relatively inexpensive and light in weight. Especially, as a container for a beverage which requires heat sterilization filling, for e.g. a fruit juice beverage, a bottle having a high gas barrier property, etc. imparted by an application of stretch heat setting, has shown a rapid expansion. Such a bottle is produced, for example, by injection molding a bottomed tubular preform, reheating the preform to soften it, followed by stretch blow molding. At that time, the blow mold is heated to apply heat setting to the bottle, whereby crystals of molecular chains aligned by stretching are fixed to provide the high gas barrier property, etc.
However, with respect to the polyethylene terephthalate resin to be used in such a field of beverage containers, in the case of a polyethylene terephthalate resin produced by using an antimony compound as a polycondensation catalyst, which is most commonly used for bottles for wide range of purpose, copolymerizable components such as isophthalic acid, diethylene glycol, etc. other than the terephthalic acid component and the ethylene glycol component, are copolymerized in an amount of from about 3 to 10 mol % based on the total dicarboxylic acid component in order to provide transparency, whereby the intended gas barrier property may not be obtained, whereby the aroma-retention property as a bottle tends to decrease, and the aroma of the content is likely to decrease, or the ultraviolet shielding property tends to be poor, whereby the flavor component or the color tone of the content is likely to deteriorate, and further, another problem is also worried such that antimony remaining in the resin will elute from the container at a high temperature and will transfer to the contained beverage although slightly. On the other hand, with a polyethylene terephthalate resin prepared by using a germanium compound as a polycondensation catalyst, which is commonly used for heat resistant bottles, copolymerizable components other than the terephthalic acid component and the ethylene glycol component, may be copolymerized in a relatively small amount at a level of more than 2 to 5 mol % based on the total dicarboxylic acid component, but the above-mentioned problem relating to a decrease of the aroma-retention property cannot still be solved, and the ultraviolet shielding property is also inferior, and further, the germanium compound is expensive, whereby an economical disadvantage cannot be avoided. Accordingly, it is strongly desired to have a substitute polycondensation catalyst developed.
Further, many polyethylene terephthalate resins have been proposed which are prepared by using titanium compounds as polycondensation catalysts, but they have had problems such that they lack in thermal stability, so that the obtainable resins tend to have a yellowish color tone, or the change in the color tone after being heated is substantial, and further have problems such that acetaldehyde, a cyclic trimer, etc. are formed in a large amount as by-products during the polycondensation and the melt molding, and when used as bottles, they tend to deteriorate the tastes, aromas, etc. of the contained beverages. Whereas, e.g. JP-A-8-73581 discloses a process for producing a polyethylene terephthalate resin which is colorless and excellent in transparency, by using a titanium compound, a cobalt compound, and a limited amount of complex-forming agent, such as phosphoric acid, phosphorous acid and/or phosphonic acid or its derivative. However, according to the study conducted by the present inventors, it has been found that the polyethylene terephthalate resin obtainable by this process is not one which is able to solve the above-mentioned problem such as a decrease in the aroma-retention property and the problem such as deterioration of the taste, the aroma, etc. of the content.
Further, EP-A-1013692 discloses that production of acetaldehyde as a by-product during the polycondensation and the melt molding can be suppressed by using titanium and metal compounds, as polycondensation catalysts, so that specific amounts of titanium atoms and metal atoms such as magnesium, would be in a specific ratio. Further, in JP-A-2000-339919 filed by the present applicants, it is disclosed that in the polycondensation in the presence of (1) a titanium compound, (2) a compound of at least one element selected from the group consisting of metal elements of Group 1A of the periodic table, elements of Group 2A of the periodic table and manganese, and (3) a phosphorus compound, the order of addition of the respective compounds (1), (2) and (3) is set to be (3), then (2) and then (1), whereby by-products such as acetaldehyde, a cyclic trimer, etc., can be reduced. However, according to the study by the present inventors, it has been found that these methods are certainly effective to reduce by-products, but with the disclosed methods, there is still a room for improvement with respect to the gas barrier property, the ultraviolet shielding property or the polycondensability.
The present invention has been made in view of the above-described prior art, and it is an object of the present invention to provide a polyester resin, whereby a molded product excellent in the gas barrier property and also excellent in the ultraviolet shielding property, the color tone, etc., and which is particularly suitable for molding a bottle for e.g. a beverage required to have an aroma-retention property, and further, a polyester resin, whereby the acetaldehyde content as a molded product is reduced to eliminate an influence over the taste, the aroma, etc. of the content, and a process for its production, whereby the polycondensability is improved.